1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same. More particularly, the present invention relates to an improved configuration of metal wiring and an improved method of fabricating the metal wiring.
2. Description of the Prior Art
Recently, as components in a semiconductor device are highly integrated, the line width of metal wiring is reduced and the aspect ratio of a contact hole is increased. Accordingly, the density of current flowing through the wiring becomes high and the step coverage of metal wiring in contact holes is deteriorated. As a result, the reliability of the wiring and the contact holes cannot be ensured.
A known wiring structure in a conventional semiconductor device which can eliminate the above disadvantage is a double-layered structure constituted by an aluminum alloy (Al-alloy) film and a barrier metal film. The double-layered structure is formed in such a manner that the barrier metal film is sputter deposited and then the Al-alloy film is deposited. As the barrier metal film, a refractory metal film or a refractory metal compound film such as a refractory metal nitride film is used.
However, when the components are more highly integrated, and hence, the size of a contact hole is reduced, the barrier metal film formed by sputtering is very thin on the bottom of the contact hole due to the poor step coverage. This causes a problem in that the reliability of contact cannot be ensured. In order to solve the problem, a method has been proposed in which a tungsten film formed by chemical vapor deposition (CVD) (hereinafter, referred to as a CVD tungsten film) is used as the barrier metal film instead of the barrier metal film formed by sputtering. The step coverage is extremely improved when utilizing CVD as compared with the case of sputtering, so that the tungsten film is thickly formed on the bottom of the contact hole, and therefore, the reliability of contact is ensured.
In order to solve the problem of poor step coverage of an Al-alloy film, another method further improved over the above-mentioned method has been proposed. In such method, contact holes are filled with tungsten by depositing a tungsten film to a thickness equal to or larger than a half length of the short side of a contact hole, and then an Al-alloy film is deposited. This improved method, however, has a disadvantage that the film thickness as wiring increases. Therefore, another method has been proposed in which a tungsten film is deposited to a thickness equal to or larger than a half length of the short side of a contact hole, and the tungsten film is used as wiring without depositing an Al alloy. This method still has a disadvantage that the wiring resistance is high because of the absence of an Al alloy. Accordingly, still another method has been reported in which a tungsten film is deposited to a thickness equal to or larger than a half length of the short side of a contact hole, and then the entire surface of the tungsten film is etched to reduce the thickness of the tungsten film. Next, an Al-alloy film is deposited, so that the total film thickness as wiring is reduced. Such a method is reported, for example, in IEEE, IEDM, Technical Digest, pp. 462-465, 1988.
An example of the above-mentioned conventional semiconductor device which is fabricated in such a manner that a tungsten film is deposited to a thickness equal to or larger than a half length of the short side of a contact hole, and then an Al-alloy film is deposited will be described below with reference to FIG. 3.
FIG. 3 is a cross-sectional view showing the main portions of the conventional semiconductor device. In FIG. 3, a first insulating film 2 is formed over a silicon substrate 1. A metal wiring 3, and then a second insulating film 4, are formed on top of the first insulating film 2. A contact hole 5 is formed in the second insulating film 4. A seed layer 6 of a titanium tungsten (TiW) film is formed on the second insulating film 4 and in the contact hole 5 as a seed for the growth of a tungsten film over the insulating film. In the case where a tungsten film is formed over an insulating film, it is necessary to form a seed layer on top of the insulating film, because a CVD tungsten film is not grown on the insulating film. Over the seed layer 6, a CVD tungsten film 7 is formed, and an Al-alloy film 9 is formed thereover.
In the semiconductor device with the above-mentioned configuration, the contact hole is filled with tungsten by forming the CVD tungsten film 7, thereby ensuring the reliability of contact. The multilayered structure of the tungsten film 7 and the Al-alloy film 9 enables the specific resistance to be lowered as compared with a single layer structure of tungsten film. Generally, the surface of the CVD tungsten film 7 is rough, as is shown in FIG. 3.
A comparison in reliability as wiring of a multilayered structure of a barrier metal film and an Al-alloy film between a case where a CVD tungsten film is used as the barrier metal film and a case where a TiW film deposited by sputtering is used as the barrier metal film is reported by H. H. Hoang et al. in Proceeding, IEEE, VMIC Conference, pp. 133-141, 1990. The report mentions that, since the surface of a CVD tungsten film is rough, in the case where the tungsten film is used as the barrier metal film, the crystal grains of the Al-alloy film are small as compared with the case of using the TiW film. Therefore, the reliability as wiring (e.g., the electromigration resistance) is poor. The report further mentions that if the Cu concentration in the Al-alloy film is 2%, there is no difference in reliability between the two cases using the different kinds of barrier metal films.
It is known that the electromigration resistance becomes high with the increase in grain diameter of an Al-alloy film. The electromigration is a phenomenon in which, when electrons flow, the electrons collide with Al atoms to move the Al atoms. It is known that, since the Al atoms are moved along the grain boundaries, in a portion A where plural grain boundaries combine into one as is shown in FIG. 4 along the direction of the flow of electrons, the Al atoms go to excess to cause hillock which results in a short circuit. In a portion B where one grain boundary divides into plural grain boundaries, the Al atoms are not sufficient so as to cause void which results in breaking of wiring. Therefore, as the crystal grain is larger, the number of grain boundaries decreases, and the electromigration resistance is enhanced.
It is also known that the electromigration resistance is enhanced when copper is added as an impurity to Al-alloy, and as the concentration of copper is higher, the effect is further enhanced. It is considered that this is because copper deposits itself on the grain boundaries and suppresses the movement of Al atoms along the grain boundaries.
However, with the above configuration using the Al-alloy film containing 2% copper, there remains a problem in corrosion of wiring. Since it is known that the corrosion of Al alloy is more likely to occur as the concentration of copper becomes higher, the Al-alloy film usually contains 0.5% copper, or 1% copper at the most. With the decrease in size of a semiconductor device, the line width of wiring is further reduced, so that the slight corrosion may cause a defect. Therefore, it becomes further difficult for the Al-alloy film to contain much copper.